1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, and particularly, to a nonreciprocal circuit device, such as an isolator and a circulator, operating in a microwave band and a communication apparatus including the nonreciprocal circuit device.
2. Description of the Related Art
In the related art, a nonreciprocal circuit device, such as an isolator or a circulator, sends signals only in a predetermined particular direction and not in a direction opposite to the predetermined particular direction. By making use of this characteristic, an isolator, for example, is used for a transmission circuit in a mobile communication apparatus, such as an automobile telephone or a cellular telephone.
Japanese Unexamined Patent Application Publication No. 2002-198707 (Patent Document 1) discloses a nonreciprocal circuit device including a ferrite core wound with copper wires that is longitudinally disposed in the perpendicular direction as a center electrode on a circuit substrate so that two permanent magnets sandwich the ferrite core.
In the nonreciprocal circuit device disclosed in Patent Document 1, however, since the ferrite core and the permanent magnets are surrounded by a magnetic yoke not only from the four sides but also from above, a DC magnetic field applied to the ferrite core from the permanent magnets is dispersed on the upper surface of the yoke. This causes a problem in that a uniform DC magnetic field cannot be applied to the ferrite core.
In addition, Patent Document 1 discloses that a hole is provided at a center portion of the upper surface of the magnetic yoke. However, since the magnetic yoke defines a magnetic circuit having a DC magnetic field, the yoke provided with the hole deteriorates uniform magnetic field strength and weakens the DC magnetic field. Furthermore, the hole is configured to include an entire planar projection area of the ferrite core, resulting in considerable leakage of a high-frequency magnetic field.